Method and apparatus of rate conversion for coded video data

ABSTRACT

Picture coded data that are obtained by coding at a coding rate R1 that is input via a coded data input terminal 1 are supplied to a variable length decoder. The variable length decoder decodes data for which variable length coding has been performed, and DCT coding information is transmitted to an inverse quantizer. A quantization controller calculates a quantization step that is necessary for re-quantization, and controls a quantizer. The DCT coding information for which the inverse quantization has been performed by the inverse quantizer is recovered into a DCT coefficient. The DCT coefficient is quantized again by a quantizer at quantization step Q that is determined by the quantization controller, and the result is transmitted to a variable length coder. The variable length coder performs variable length coding of the received data at a coding rate R2, and outputs the resultant picture coded data to a coded data output terminal. In this manner, a rate conversion apparatus for picture coded data can be provided that employs simpler means than does the transcoding that provides the same performance as does transcoding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for rate conversion of codedvideo data and to an apparatus for the performance of the conversion;and relates in particular to a method, for rate conversion of codedvideo data, whereby an apparatus that records, transfers, and displays adigital moving picture requires but a simple arrangement to perform rateconversion which involves little degradation of pictures, and to anapparatus by which the method is applied.

2. Description of the Related Art

Conventional examples of highly efficient coding for moving pictures arestandards methods for moving picture coding that are proposed by the ISO(International Organization For Standardization) and the JTC (JointTechnical Committee) of the IEC (International ElectrotechnicalCommission), i.e., MPEG1 (Moving Pictures Experts Group) and MPEG2. Whenthe MPEG coding system is employed, a television signal is convertedinto coded data with a transfer speed of 1 Mbit/s to several tens ofMbit/s, in consonance with a picture quality, and is accumulated on acomputer hard disk or is transmitted over a LAN (Local Area Network).

Since usable transfer speeds vary across a transfer path, such as theInternet or an ISDN, according to the busyness of lines and thecontracted line speed, requests for the transfer of moving pictures aremade at various speeds. Therefore, there is one technique, for example,whereby when coded data that are temporarily accumulated are present,the data are decoded to recover a picture and coding is again performedin consonance with a required transfer speed.

This technique is called a transcoding system. The outline of thetranscoding system will be explained while referring to FIG. 11.

As is shown in FIG. 11, data that are coded at a coding rate R1 that isinput at a coded data input terminal 51 are temporarily decoded into apicture by a decoder 52. The picture is then transmitted to a coder 53,and is coded again at a coding rate that is input via a coding rateinput terminal 55. The coded data are output, via a coded data outputterminal 54, as data whose coding rate has been changed to R2.

On the other hand, there are techniques whereby coding rate conversionis performed on coded data without decoding coded data into a picture.One such techniques is described in "The Study Of A Coding RateConversion System," by Matsumoto and Kimura, 1994 ITEJ (The Institute ofTelevision Engineers of Japan) Annual Convention, pp. 183 to 184. Thissystem will be briefly explained while referring to FIG. 12.

As is shown in FIG. 12, a variable length decoder 62 extracts DCT(Discrete Cosine Transform) coding information from data that have beencoded at a coding rate R1 that is input via a coded data input terminal61. One part of the DCT coding information is selected and the remainderis deleted by a DCT coefficient selector 63 in accordance with a codingrate that is input via a coding rate input terminal 66. The selected DCTcoding information is coded by a variable length coder 64, and theresultant data is output, via a coded data output terminal 65, as codeddata whose coding rate has been changed to R2. This system is called DCTcoefficient partitioning.

The DCT coefficient partitioning will be described in detail whilereferring to FIG. 13. Although a coding process is generally performedwhile employing an 8×8 pel grid as one block, for simplicity a 4×4 pelblock is shown in FIG. 13. In FIG. 13, assuming that reference number 71denotes a coefficient matrix that is obtained by performing a DCTprocess on a 4×4 pel block (not shown), the elements of the coefficientmatrix 71 are divided by corresponding elements in a quantization matrix72. The remainder of this division is omitted, and a quantizedcoefficient matrix 74 (i.e., the above described coded information) isacquired from the integer portion. The elements of the coded information74 are rearranged in the order of the elements in a zig-zag scan matrix75, and thus a quantized coefficient matrix 76 is acquired. Then,entropy coding is performed on the quantized coefficient matrix 76.

According to the DCT coefficient partitioning, the variable lengthdecoder 62 decodes the data acquired by performing entropy coding whilethe DCT coefficient selector 63 selects, for example, only a part 74a ofthe coded information 74 and deletes the remainder of the codedinformation. In this manner, the DCT coded information 74a that wasselected is coded by the variable length coder 64, and the resultantdata are output, via the coded data output terminal 65, as coded datawhose coding rate has been changed to R2.

According to the description in the above document, however, assumingthat the original coding rate R1 is 6 Mbit/s and the converted rate R2is 4 Mbit/s, the transcoding in MPEG2 is degraded by about 1.5 dB ascompared with normal coding at 4 Mbit/s. The term normal coding meansthe coding that is ordinarily performed without rate conversion, i.e.,the coding system in which a picture is directly coded by a coder. Thetranscoding requires the decoder 52 and the coder 53, so that the sizeof the arrangement for an apparatus is increased and the manufacturingcost of the apparatus is also raised.

Since the DCT coefficient partitioning can be obtained by using thevariable length decoder 62, the DCT coefficient selector 63, and thevariable length coder 64, the arrangement for an apparatus is verysimple. According to the description in the above document, however, theDCT coefficient partitioning is degraded by about 3 dB compared with theresults that are obtained with normal coding. Therefore, with the DCTcoefficient partitioning, conversion causes a picture to be deterioratedvery much. The cause of the deterioration in a picture is regarded asbeing the deletion of information other than the part 74a of the DCTcoded information 74 in FIG. 13, for example.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a method, for therate conversion of video data, whereby the above described shortcomingsof the prior art are overcome and a means that is simpler thantranscoding is employed to provide the same performance as that which isprovided by transcoding, and to provide an apparatus for the employmentof the method.

It is another object of the present invention to provide a method, forthe rate conversion of video data, whereby a means that is simpler thantranscoding is employed to obtain a higher efficiency for rateconversion than that which is provided by DCT coefficient partitioning,and to provide an apparatus for the employment of the method.

To achieve the above objects, according to the present invention, (1) amethod, for the rate conversion of video coded data, comprises the stepsof re-quantizing coded data obtained by performing inverse quantization,and of performing rate conversion at a quantization level; and (2) anapparatus, for the rate conversion of video data, comprises means forinversely quantizing coded data for a moving picture, means forre-quantizing coded data obtained by inverse quantization, andquantization control means for controlling a quantization step that isrequired for said re-quantization.

According to the present invention, since the rate conversion of codedvideo data can be performed without deleting the coded video data, arate for coded data can be converted with minimum deterioration of apicture. Further, since the decoder and the coder that are employed in aconventional apparatus are not required, an apparatus that has a simplestructure can be manufactured at a low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the arrangement of an apparatusfor coding rate conversion according to one embodiment of the presentinvention;

FIG. 2 is a block diagram illustrating the arrangement of a firstembodiment of a quantization controller in FIG. 1;

FIG. 3 is a block diagram illustrating the arrangement of a secondembodiment of the quantization controller in FIG. 1;

FIG. 4 is a block diagram illustrating the arrangement of a thirdembodiment of the quantization controller in FIG. 1;

FIG. 5 is a graph showing a relationship between a block activity and acoded block count;

FIG. 6 is an explanatory graph for acquiring a block activity from acoded block count by referring to FIG. 5;

FIG. 7 is a block diagram illustrating the arrangement of a fourthembodiment of the quantization controller in FIG. 1;

FIG. 8 is an explanatory graph for acquiring a quantization step from ablock activity and a coded block count by referring to FIG. 5;

FIG. 9 is a diagram for explaining bit quantities that are employed forcomputation of the quantization controller;

FIGS. 10A and 10B are diagrams for explaining the arrangement of a macroblock;

FIG. 11 is a block diagram illustrating the arrangement of a coding rateconversion apparatus as prior art 1;

FIG. 12 is a block diagram illustrating the arrangement of a coding rateconversion apparatus as prior art 2; and

FIG. 13 is a diagram for explaining the outline of a conventional codingprocess.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail while referring tothe accompanying drawings. Although the present invention can be appliedto any video coding system for either still pictures or for movingpictures, in the following embodiment the MPEG coding method for movingpictures that uses the DCT will be explained. With MPEG, the DCT codingis performed for individual blocks of 8 pels × 8 lines. The quantizationstep is provided for each macro block. A macro block in MPEG1 consistsof, for example, 16 pels × 16 lines, as is shown in FIG. 10A, and isconstituted by four luminance blocks of 8 pels × 8 lines each and twochroma blocks of 8 pels × 8 lines.

FIG. 1 is a block diagram illustrating a first embodiment of a rateconversion apparatus according to the present invention. Coded videodata la that are input via a coded data input terminal 1 and that arecoded at coding rate R1 are first transmitted to a variable lengthdecoder 2. The variable length decoder 2 decodes the data for whichvariable length coding was performed. Here, DCT coding information 2aand quantization step information for each block are decoded. The DCTcoding information 2a is forwarded to an inverse quantizer 3, whileinformation (e.g., quantization step information and a motion vector)2b, which is information other than the DCT coding information, is sentto a memory 4. The variable length decoder 2 counts the total bit count2c in each picture, and the total bit count 2c is transmitted to aquantization controller 5.

The inverse quantizer 3 performs inverse quantization on the receivedDCT coding information 2a to recover a DCT coefficient 3a. The DCTcoefficient 3a corresponds to the DCT coefficient matrix 71 in FIG. 13.The recovered DCT coefficient 3a is quantized again by a quantizer 6 ata quantization step Q that is determined by the quantization controller5. The quantization controller 5 receives a quantization step 4a fromthe memory 4; the total bit count 2c before conversion from the variablelength decoder 2; a total bit count 7b up to a conversion block afterconversion and a quantization step 7c after conversion from a variablelength coder 7; and a coding rate 9a from a coding rate input terminal9.

DCT coding information 6a that is output by the quantizer 6 istransmitted to the variable length coder 7 with information 4b, such asa motion vector that is stored in the memory 4, that is other than theDCT coding information and the quantization step Q from the quantizationcontroller 5. After variable length coding is performed on the DCTcoding information 6a, the resultant information is output, from a codeddata output terminal 8, as coded video data 7a that is obtained bycoding at a coding rate R2. Reference numbers 11, 14, 16, 19, 20 and 22are provided in FIG. 1 in order to clarify their relationship with thelike numbers in FIG. 2.

The same system as is used for a decoder that conforms to the MPEGsystem can be employed for the variable length decoder 2 and the inversequantizer 3. The same system as is used for a coder that conforms to theMPEG system can be employed for the quantizer 6 and the variable lengthcoder 7.

FIG. 2 is a block diagram illustrating a first embodiment of thequantization controller 5. The same reference numbers as used in FIG. 1are also used in FIG. 2 to denote corresponding or identical components.Pre-conversion quantization step Q1m(n,i) for a block that is currentlybeing processed is input at a quantization step input terminal 11. The ndefines a frame number, and the numbering is performed in consonancewith the display order for the pictures. The i defines the number of amacro block, and as is shown in FIGS. 10A and 10B, for example, thenumbers are provided from the left top to the right bottom of a pictureP (i=0, 1, 2, . . . T-1, where T is a complete macro block in thepicture). The definitions for i and n can be applied hereafter in thesame manner.

The quantization step Q1m(n,i) is sent to an average value processingsection 12 as well as a quantization step computation section 21. Theaverage value processing section 12 adds together the quantization stepsof the individual macro blocks in the picture, and calculates theiraverage value when all the picture quantization steps are received toacquire picture averaged quantization step Q1p(n). The picture averagedquantization step Q1p(n) that is output by the average value processingsection 12 is stored in a memory 13. From the memory 13 is outputaveraged quantization step Q1p(n-k) for the latest coded picture withthe same coding type. In this case, k defines a positive integer. Framenumber n-k defines a number for the latest decoded picture with the samecoding type as a currently processed picture n. For example, when theframe number of a currently processed picture is n=5 and the coding typeis a predictive coded picture (P picture) type that is specified by theMPEG, and when the frame of the latest coded picture P is presentpreceding two frames, k=2 and accordingly n-k is 3, which indicatesframe number 3.

Total coded bit count B1p(n) in a picture before conversion is input ata pre-conversion picture bit count input terminal 14. The coded bitcount B1p(n) is stored in the memory 15. From the memory 15 is outputtotal coded bit count B1p(n-k) for the latest coded picture with thesame coding type before conversion.

Post-conversion quantization step Q2m(n,i) at a post-conversionquantization step input terminal 16 is transmitted to an average valueprocessing section 17. The average value processing section 17 addstogether the quantization steps for the macro blocks in the picture andcalculates the average value when all the quantization steps in thepicture are received to provide average quantization step Q2p(n) for thepicture after conversion. The average quantization step Q2p(n) for thepicture after conversion, which is output from the average processingsection 17, is stored in a memory 18. From the memory 18 is outputaverage quantization step Q2p(n-k) for the latest coded picture afterconversion that is the same coding type.

Coded bit count B2u(n,i) up to the blocks for the picture afterconversion is input at a post-conversion picture bit count inputterminal 19.

The quantization step computation section 21 receives the quantizationstep Q1m(n,i) for a block from the quantization step input terminal 11,the picture average quantization step Q1p(n-k) from the memory 13, thepre-conversion picture coded bit count B1p(n-k) from the memory 15, thepost-conversion picture average quantization step Q2p(n-k) from thememory 18, the coded bit count B2u(n,i), which is up to the block in thepicture after conversion, from the post-conversion picture bit countinput terminal 19, and the coding rate R1 before rate conversion and thecoding rate R2 after conversion from the coding rate input terminal 20.The quantization step computation section 21 transmits quantization stepQ2m(n,i) after conversion to a quantization step output terminal 22. Thenumber "1" in Q1m, B1p, etc., designates a condition before conversion,while the number "2" in Q2p and B2u designates a condition afterconversion. These conditional designations can be employed hereafter.

The processing at the quantization step computation section 21 will nowbe explained in detail. First, the definitions of primary counts thatappear in the following explanation will be described while referring toFIG. 9. Suppose that the picture is changed to P(n-k), P(n-k+1), . . . ,P(n) as time elapses and that the macro block in the picture for whichrate conversion is currently being performed is the ith block i inpicture P(n) of the nth frame. Also, suppose that the latest codedpicture with the same coding type as picture P(n) is picture P(n-k)which precedes picture P(n) by k frames. The definitions of the primarycounts are as follows:

Q1p(n-k) . . . average quantization step for picture P(n-k) before rateconversion;

Q2p(n-k) . . . average quantization step for picture P(n-k) after rateconversion;

B1p(n-k) . . . total bit count of picture P(n-k) before rate conversion;

Q1m(n,i) . . . quantization step for macro block i in picture P(n)before rate conversion;

Q2m(n,i) . . . quantization step for macro block i in picture P(n) afterrate conversion;

Q2b(n,i) . . . base quantization step for macro block i in picture P(n)after rate conversion;

B1p(n) . . . total coded bit count for picture P(n) before conversion;

B2t(n) . . . target bit count for picture P(n) after rate conversion;

B2u(n,i) . . . bit count up to macro block i (shaded portion in thedrawings) in picture P(n) after rate conversion; and

η. . . target bit count of picture P(n) reflecting coded bit count up tomacro block i (shaded portion in the drawings) in the picture P(n) afterrate conversion.

The quantization step computation section 21 employs the basequantization step Q2b(n,i) for the block, the quantization step Q1m(n,i)for the block before conversion, and the average quantization stepQ1p(n-k), before conversion, for the latest coded picture with the samecoding type as a current picture (hereafter referred to as a "latestcoded picture with the same coding type") to acquire the quantizationstep Q2m(n,i).after conversion by function F1 as follows:

    Q2m(n,i)=F1(Q2b(n,i), Q1m(n,i), Q1p(n-k))                  Eq. 1

Supposing that scene is continuous, e ratio of the pre-conversionquantization step Q1m(n,i) for the block i to the pre-conversion averagequantization step Q1p(n-k) for the latest coded picture with the samecoding type means the characteristic of the average quantization in thepicture for the block 1. For example, when the quantization stepQ1m(n,i) is great relative to the average quantization step Q1p(n-k),the quantization step Q2m(n,i) after the conversion must be greater thanthe average quantization step after the conversion. Therefore, as isshown by equation 2 below, the function F1 can be represented asmultiplication of the base quantization step Q2b(n,i) for the block i bythe weighted coefficient Q1m(n,i)/Q1p(n-k). As is apparent from equation4, the base quantization step Q2b(n,i) for the block i is a quantizationstep where the coded bit count up to the block i after the conversion isreflected in the average quantization step Q2p(n-k), after theconversion, for the latest coded picture with the same coding type.

    F1(Q2b(n,i), Q1m(n,i), Q1p(n-k,i)) =αQ2b(n,i)×Q1m(n,i)/Q1p(n-k) Eq. 2

where α is a positive real number, and for example, α=1.

The base quantization step Q2b(n,i) for the block can be calculated withfunction G1 by employing the quantization step Q2p(n-k), after theconversion, for the latest coded picture with the same coding type, thetarget bit count B2t(n) for the picture, and the bit count B2u(n,i) upto the block after the conversion.

    Q2b(n,i)=G1(Q2p(n-k), B2t(n), B2u(n,i))                    Eq. 3

B2t(n)/η, the ratio of the target bit count B2t(n) for the picture tothe target bit count η for the picture that reflects the coded bit countup to the block after the conversion, indicates the characteristic ofthe quantization that reflects a bit count that can be used in theblock. When, for example, η is greater than B2t(n), the already codedbit count is greater than is expected, and thus the base quantizationstep must be increased. As the base of the quantization step after theconversion, the average quantization step Q2p(n-k), after theconversion, of the latest coded picture with the same coding type can beused. Therefore, as is shown by equation 4, the function G1 can bedepicted as the multiplication of the quantization step Q2p(n-k) by theweighted coefficient B2t(n)/η:

    G1(Q2p(n-k), B2t(n), B2u(n,i)) =Q2p(n-k)×B2t(n)/ηEq. 4

The change in the used bit count up to the block i is obtained fromB2t(n)xi/T-B2u(n,i), while yields the difference between the bit countB2t(n)xi/T, for use up to the block i, and the bit count B2u(n,i), whichis obtained by actual coding. When, for example, B2t(n) is 100 kbits, iis 20, and T is 40, the bit count that is to be used is 50 kbits. Whenthe bit count B2u(n,i) that is obtained by actual coding is 60 kbits, itindicates that the actual coded bit count has exceeded the bit countthat is to be used. Therefore, as is shown by equation 5, η can beacquired by adding the change in the value of the used bit count up tothe block to the target bit count B2t(n) for the picture:

    η=(B2t(n)+B2t(n)xi/T-B2u(n,i))                         Eq. 5

Further, B2t(n) can be acquired as follows, with function H1 byemploying the bit count B1p(n-k), for the latest coded picture with thesame coding type as a current picture, and the coding rates R1, beforethe conversion, and R2, after the conversion:

    B2t(n)=H1(B1p(n-k), R1, R2)                                Eq. 6

The function H1 can be obtained by multiplying the total bit countB1p(n-k) of the latest coded picture with the same coding type by rateconversion ratio R2/R1:

    H1(B1p(n-k), R1, R2)=εB1p(n-k)×R2/R1         Eq. 7

where ε is a positive real number and, for example, ε=1.

As the conclusion, the quantization step Q2m(n,i) after the conversioncan be acquired with equation 8, as follows, by employing the quantitiesthat are received by the quantization step computation section 21 inFIG. 2: ##EQU1## It should be noted that B2t(n)=εB1p(n-k)×R2/R1.

According to this embodiment, in the above described manner, thequantization step Q2m(n,i) for the block i, after the conversion that isoutput by the quantization step computation section 21, can be acquiredby using data that are input to the quantization step computationsection 21. In this embodiment, unlike the conventional DCT coefficientpartitioning where a part of the DCT coding information is deleted, theinverse quantization is performed for the DCT coding information by theinverse quantizer 3, and rate conversion is performed for the resultantinformation based on the quantization step Q, which is obtained by thequantization step computation section 21. Thus, the rate conversion canbe performed with less image deterioration than in the DCT coefficientpartitioning. In addition, the quantization step computation section 21acquires the quantization step Q for the block i of the frame n, whiletaking into account the bit count up to the block i in the frame n thatthe block i belongs to, and the quantities of latest coded pictures(n-k) with the same coding type. Therefore, efficient rate conversionthat matches the requirement of a system can be performed.

A second embodiment of the present invention will now be described whilereferring to FIG. 3. FIG. 3 is a block diagram illustrating a secondembodiment of the quantization controller 5 in FIG. 5. The samereference numbers as used in FIG. 2 are used to denote corresponding oridentical components.

In FIG. 3, coded bit count B1p(n), for the picture before theconversion, and coded bit count B1m(n,i), for the block i before theconversion, are input at a pre-conversion picture bit count inputterminal 14. These data are stored in a memory 15. Quantization stepafter conversion, Q2m(n,i), is input via a post-conversion quantizationstep input terminal 16 and from a memory 18 is output averagedquantization step Q2p(n-k), after the conversion, of the latest codedpicture with the same coding type.

Coded bit count B2u(n,i), up to the block i of the picture after theconversion, is input via a post-conversion picture bit count inputterminal 19. A quantization step computation section 21 receives, fromthe memory 15, the coded bit count B1p(n-k), for the picture before theconversion, and the bit count B1m(n,i), for the block i before theconversion; from the post-conversion picture bit count input terminal19, the coded bit count B2u(n,i), up to the block i, for the pictureafter the conversion; and, from a coding rate input terminal 20, codingrate R1, before the conversion, and coding rate R2, after theconversion. The quantization step computation section 21 outputspost-conversion quantization step Q2m(n,i) to a quantization step outputterminal 22.

The quantization step computation section 21 employs the basequantization step Q2b(n,i) for the block i, the bit count B1m(n,i) forthe block i before the conversion, and the bit count B1p(n-k), beforethe conversion, of the latest coded picture with the same coding type toacquire the quantization step Q2m(n,i) after the conversion by usingfunction F3 as follows:

    Q2m(n,i)=F3(Q2b(n,i), B1m(n,i), B1p(n-k))                  Eq. 9

T×B1m(n,i)/B1p(n-k), the ratio of the bit count of a picture, which is Ttimes the bit count B1m(n,i) for the block i before conversion, to thebit count B1p(n-k), before the conversion of the latest coded picturewith the same coding type shows the characteristic of the quantizationthat reflects the bit count of the block i. When, for example,T×B1m(n,i) is greater than B1p(n-k), the coded bit count of the block isgreater than average, and thus the quantization step must be increased.Therefore, the function F3 can be represented as the multiplication ofthe base quantization step Q2b(n,i) by the weighted coefficientT×B1m(n,i)/B1p(n-k), as is shown by equation 10:

    F3(Q2b(n,i), B1m(n,i), B1p(n-k)) =βQ2b(n,i)×T×B1m(n,i)/B1p(n-k)           Eq. 10

where β is a positive real number and, for example, β=1; and where Tindicates the total block count in the picture. The base quantizationstep Q2b(n,i) for the block i can be acquired in the same manner as inthe first embodiment.

The second embodiment differs from the first embodiment in that the basequantization step Q2b(n,i) is multiplied by the weighted coefficientT×B1m(n,i)/B1p(n-k). As well as the first embodiment, however, the rateconversion can be performed with less image deterioration than thatwhich occurs with the DCT coefficient partitioning.

A third embodiment of the present invention will now be described whilereferring to FIG. 4. FIG. 4 is a block diagram illustrating a thirdembodiment of the quantization controller 5 in FIG. 1. The samereference numbers as used in FIG. 2 are employed to denote correspondingor identical components.

In FIG. 4, quantization step Q1m(n,i) for a block that is currentlybeing processed is input at a quantization step input terminal 11. Thequantization step Q1m(n,i) is transmitted not only to a quantizationstep computation section 21 but also to an average value processingsection 12. The average value processing section 12 adds together thequantization steps of the blocks in the picture, and calculates anaverage value when all the quantization steps in the picture arereceived, so that average quantization step Q1p(n) for the picture isacquired. The picture average quantization step Q1p(n) is transmittedfrom the average value processing section 12 and stored in a memory 13.

Coded bit count B1p(n) for the picture before the conversion and codedbit count B1m(n,i) for the block i are input at a pre-conversion picturebit count input terminal 14. These data are stored in a memory 15.Quantization step Q2m(n,i), after the conversion, is input at apost-conversion quantization step input terminal 16. From a memory 18 isoutput average quantization step Q2p(n-k), after the conversion, of thelatest coded picture with the same coding type. Coded bit countB2u(n,i), up to the block i in the picture after the conversion, isinput at a post-conversion picture bit count input terminal 19.

A quantization step computation section 21 receives, from thequantization step input terminal 11, the block quantization stepQ1m(n,i); from the memory 13, the picture average quantization stepQ1p(n-k); from the memory 15, the coded bit count B1p(n-k) for thepicture before the conversion and the bit count B1m(n,i) for the block ibefore the conversion; from the post-conversion picture bit count inputterminal 19, the coded bit count B2u(n,i), up to the block i, for thepicture after the conversion; and, from a coding rate input terminal 20,coding rate R1 before the conversion and coding rate R2 after theconversion. The quantization step computation section 21 outputspost-conversion quantization step Q2m(n,i) to a quantization step outputterminal 22.

The quantization step computation section 21 employs the basequantization step Q2b(n,i) for the block i, estimated activity Am(n,i)for the block i, and estimated activity Ap(n-k) for each block in thelatest coded picture with the same coding type to acquire thequantization step Q2m(n,i) after the conversion by using function F4 asfollows:

    Q2m(n,i)=F4(Q2b(n,i), Am(n,i), Ap(n-k))                    Eq. 11

Am(n,i)/Ap(n-k), the ratio of the estimated activity Am(n,i) for theblock i to the estimated activity Ap(n-k) for each block in the latestcoded picture with the same coding type indicates the characteristic ofthe quantization that reflects the activity of the block i. When, forexample, the activity of the block i, Am(n,i), is greater than thepicture-averaged activity Ap(n-k), the coded bit count for the block isgreater than average, and thus the quantization step must be increased.Therefore, the function F4 can be represented as the multiplication ofthe base quantization step Q2b(n,i) by the weighted activity coefficientAm(n,i)/Ap(n-k), as is shown by equation (12):

    F4(Q2b(n,i), Am(n,i), Ap(n-k)) =γQ2b(n,i)×Am(n,i)/Ap(n-k) Eq. 12

where γ is a positive real number and, for example, γ=1.

The base quantization step Q2b(n,i) for the block i can be acquired inthe same manner as in the first embodiment. Further, the estimatedactivity Am(n,i) of the block i can be acquired with function H1 byemploying the quantization step Q1m(n,i) for the block i before theconversion and the bit count B1m(n,i) for the block i before theconversion:

    Am(n,i)=H1(Q1m(n,i), B1m(n,i))                             Eq. 13

The estimated activity Ap(n-k) for each block in the latest codedpicture can be acquired with function H2 by employing the averagequantization step Q1p(n-k) before the conversion for the latest codedpicture, and the total bit count B1p(n-k) for that picture:

    Ap(n-k)=H1(Q1p(n-k), B1p(n-k)/CB)                          Eq. 14

where CB indicates the number of blocks that are coded in the picture.

One example graph of the functions H1 and H2 is shown in FIG. 5. Thisgraph shows the relationship between the activity for each block and thebit count for each block by employing the quantization step as aparameter. FIG. 6 is an example graph for acquiring Am(n,i) by using therelationship shown in FIG. 5. First, acquired as the estimated activityAm(n,i) for the block is the activity at the intersection of the bitcount B1m(n,i), for the block before the conversion, on the graph of thequantization step that corresponds to the quantization step Q1m(n,i),before the conversion. Acquired as the estimated activity Ap(n-k), foreach block of the latest coded picture, is the activity at theintersection of the average bit count B1p(n-k)/CB, for the latest codedpicture, on the graph that corresponds to the average quantization stepQ1p(n-k) for that picture before conversion.

The graph shown in FIG. 5 can, for example, be obtained as follows.First, the quantization step is fixed at the available minimum value anda picture is coded. At this time, the generated bit count and theactivity for a block are measured and sorted according to picture codingmodes (I, P, and B pictures) that are used by the MPEG. The activity canbe acquired, for example, as a mean square error of a luminance signalin a block with resect to the average luminance in the block. Themeasuring of the activity relative to the generated bit count isperformed for a plurality of pictures and a graph of the bit countrelative to the activity is prepared for each coding mode. Then, thequantization step is increased, and the same process is performed toprepare a graph concerning the quantization step. This process isrepeated up to the maximum quantization step.

A fourth embodiment of the present invention will now be described whilereferring to FIG. 7. FIG. 7 is a block diagram illustrating a fourthembodiment of the quantization controller 5 in FIG. 1. The samereference numbers as used in FIG. 2 are employed to denote correspondingor identical components.

In FIG. 7, quantization step Q1m(n,i) for a block that is currentlybeing processed is input via a quantization step input terminal 11.Coded bit count B1m(n,i) for a block 1 before the conversion is inputvia a pre-conversion picture bit count input terminal 14. A quantizationstep computation section 21 receives the quantization step Q1m(n,i) forthe block from the quantization step input terminal 11; the coded bitcount B1m(n,i) for the block i, before the conversion, from thepre-conversion picture bit count input terminal 14; and coding rate R1,before the conversion, and coding rate R2, after the conversion from acoding rate input terminal 20. The quantization step computation section21 outputs post-conversion quantization step Q2m(n,i) via a quantizationstep output terminal 22.

The quantization step computation section 21 employs estimated activityAm(n,i) for the block i and estimated bit count B2m(n,i) for the block iafter the conversion to acquire the post-conversion quantization stepQ2m(n,i) by using function F5 as follows:

    Q2m(n,i)=F5(Am(n,i), B2m(n,i))                             Eq. 15

The estimated activity Am(n,i) for the block i can be acquired usingfunction H1 by employing the quantization step Q1m(n,i) for the block ibefore the conversion, and the bit count B1m(n,i) for the block i beforethe conversion as follows:

    Am(n,i)=H1(Q1m(n,i), B1m(n,i))                             Eq. 16

The estimated bit count B2m(n,i) for the block i after the conversioncan be acquired by employing the bit count B1m(n,i) of the block ibefore the conversion, the coding rate R1 before the rate conversion,and the coding rate R2 after the rate conversion as follows:

    B2m(n,i)=δB1m(n,i)×R2/R1                       Eq. 17

where δ indicates a positive real number and, for example, δ=1.

The functions F5 and H1 can be acquired by referring to FIG. 5 as anexample. FIG. 8 is a graph for acquiring Am(n,i) and Q2m(n,i) using therelationship shown in FIG. 5. First, acquired as the estimated activityAm(n,i) for the block is the activity at the intersection of the bitcount B1m(n,i) for the block, before the conversion, on the graph of thequantization step that corresponds to the quantization step Q1m(n,i),before the conversion. Then, acquired as the quantization step after theconversion, Q2m(n,i), is a quantization step at the intersection, ornearby, that is formed with the estimated activity Am(n,i) and theestimated bit count B2m(n,i).

(Modification)

The present invention is not limited to the above described embodiments,but can be variously modified. First, when the variable length codingprocess is not performed after the quantization process, and when theother coder and decoder are provided for coding, the variable lengthcoder and the variable length decoder in the example arrangement in FIG.1 can be replaced by a coder and a decoder.

In addition, the present invention can be employed for moving picturecoding systems and still picture coding systems, such as the H.261system of the ITU-T and the JPEG system, other than the MPEG.

Further, as for the picture average quantization step Q1p(n-k) that isacquired in frame n-k, which indicates the number of the latest codedpicture, the coded bit count B1p(n-k) for the picture before theconversion, and the estimated activity Ap(n-k, i) for each block for thelatest coded picture, k=1 that indicates the latest frame and k=0 thatindicates the current frame can be employed for the purpose ofsimplifying the processing.

Q1m(n,i)/Q1p(n-k) in equation 2 can be changed to an easy term as afunction for calculating the quantization step. For example, constant"1" can be used as a simple weighted value. At this time, the functionfor calculating the quantization step Q2m(n,i) is the function for thebase quantization step Q2b(n,i). In this case, the performance after therate conversion may be slightly deteriorated compared with that in theabove described embodiments.

Although in the graph in FIG. 5, the relationship between the bit countand the activity in FIG. 5 is shown for each picture coding mode (I, P,or B picture) that is used in the MPEG, for example, a graph for aspecific coding mode can be employed as a representative model, or anaveraged graph of all the coding modes can be used as a representativemodel. When a system other than the MPEG is employed, a graph can beprepared according to that system.

When the variable length decoder 2 and the variable length coder 7 inFIG. 1 are changed so as to enable them to cope with a different system,they can be used as system converters that perform conversion fordifferent systems and rate conversions. For example, data that are codedat 2 Mbit/s by the MPEG2 system can be converted into data at 1 Mbit/s,which are in turn changed into data at 1 Mbit/s for the MPEG1 system bya variable length decoder of the MPEG1 system, or the coded data at 1Mbit/s for the MPEG1 system can be converted into data at 1 Mbit/s forthe H.261 system.

As is apparent from the above description, according to the presentinvention, since rate conversion is basically performed by employing thequantizer and the inverse quantizer, the DCT and the IDCT, which requirean enormous amount of processing, are not necessary, and as a memorywith a large capacity is not therefore required, the present inventioncan be achieved with a simple structure. The performance of the presentinvention is the equivalent of transcoding. Since the DCT coefficientpartitioning does not require quantization and inverse quantization,slightly less processing is required for the employment of that systemthan is required with the present invention; however, the performance ofthe DCT coefficient partitioning is substantially degraded compared withthe present invention.

In the present invention, a picture that was coded with the MPEG1 systemwas processed. When the processing in the first embodiment was performedfor "Flower Garden" and "Mobile Calendar," which are used as testpictures by the ISO, it was confirmed that, while the amount ofprocessing was considerably reduced compared with that which is requiredfor that transcoding during which data are decoded to obtain a pictureand then the picture is coded again, an SN ratio that is substantiallythe same as that for transcoding could be acquired. In addition, whencompared with the DCT coefficient partitioning, it was confirmed that,although the amount of processing was not much different, the codingperformance could be improved by up to 8 dB.

What is claimed is:
 1. A coding rate conversion method, for converting acoding rate of a moving picture data coded once,wherein the movingpicture data is decoded by a variable-length decoder providing decodeddata, said decoded data is inversely quantized providing inverselyquantized data, said inversely quantized data is subject to quantizationstep control providing re-quantized data, and said re-quantized data isvariable-length coded, whereby a rate conversion is performed at aquantization level.
 2. In a coding rate conversion apparatus, forconverting a coding rate of moving picture data coded once,means forconverting the coding rate of coded data for the moving picturecomprising:means for decoding the coded data for the moving picture by avariable-length decoder; means for inversely quantizing the data decodedby said variable-length decoder; means for re-quantizing said inverselyquantized coded data; and quantization control means for controlling aquantization step that is required for said re-quantization.
 3. A rateconversion apparatus according to claim 2, wherein said quantizationcontrol means controls said re-quantization of a target block by usingan average quantization step before and after a conversion of a picturethat is coded in a like manner or is already coded.
 4. A rate conversionapparatus according to claim 2, wherein said quantization control meanscontrols said re-quantization of said target block by employing a basequantization step for said target block that is obtained by reflecting,at said average quantization step after said conversion of said picturethat is coded in a like manner or is already coded, a bit count aftersaid conversion up to said target block in a frame for a moving image.5. A rate conversion apparatus according to claim 2, wherein, accordingto a ratio of said quantization step, for said target block before saidconversion, to said average quantization step before said conversion ofsaid picture that is coded in a like manner or is coded already, saidquantization control means weighs said base quantization step for saidtarget block, which is obtained by reflecting, at said averagequantization step after said conversion of said picture that is coded ina like manner or is coded already, said bit count after a conversion upto said target block in said frame of said moving picture, and thusacquires a quantization step that is necessary for re-quantization so asto control said re-quantization of said target block.
 6. A rateconversion apparatus according to claim 2, wherein, according to a ratioof a bit count, before said conversion of said picture that is coded ina like manner or is coded already, to a bit count that is obtained bycalculating for a picture a bit count of said target block before saidconversion, said quantization control means weighs said basequantization step for said target block, which is obtained byreflecting, at said average quantization step after said conversion ofsaid picture that is coded in a like manner or is coded already, saidbit count after said conversion up to said target block in said frame ofsaid moving picture, and thus acquires a quantization step that isnecessary for re-quantization so as to control said re-quantization ofsaid target block.
 7. A rate conversion apparatus according to claim 2,wherein, according to a ratio of an estimated activity for said targetblock, to an estimated activity for each block of said picture that iscoded in a like manner or is coded already, said quantization controlmeans weighs said base quantization step for said target block, which isobtained by reflecting, at said average quantization step after saidconversion of said picture that is coded in a like manner or is codedalready, said bit count after said conversion up to said target block insaid frame of said moving picture, and thus acquires a quantization stepthat is necessary for re-quantization so as to control saidre-quantization of said target block.
 8. A rate conversion apparatusaccording to claim 7, wherein said estimated activity for each block,for said picture that is coded in a like manner or is coded already, isacquired by said average quantization step before said conversion ofsaid picture that is coded in a like manner or is coded already and atotal bit count for said picture, and wherein said estimated activity,for said target block, is acquired by said quantization step for saidtarget block before said conversion and said bit count for said targetblock before said conversion.
 9. A rate conversion apparatus accordingto claim 2, wherein, for control of said re-quantization of said targetblock, said quantization control means acquires said quantization step,which is necessary for said re-quantization by said estimated activityand said estimated bit count after said conversion of said target blockin said frame of said moving picture.